{config, pkgs, ...}:

with pkgs.lib;

let

  upstart = pkgs.upstart;


  # From a job description, generate an Upstart job file.
  makeJob = job:

    let

      jobText =
        ''
          # Upstart job `${job.name}'.  This is a generated file.  Do not edit.
        
          description "${job.description}"

          ${if isList job.startOn then
              # This is a hack to support or-dependencies on Upstart 0.3.
              concatMapStrings (x: "start on ${x}\n") job.startOn
            else if job.startOn != "" then
              "start on ${job.startOn}"
            else ""
          }
          
          ${if job.stopOn != "" then "stop on ${job.stopOn}" else ""}

          ${concatMapStrings (n: "env ${n}=${getAttr n job.environment}\n") (attrNames job.environment)}
          
          ${if job.preStart != "" then ''
            start script
              ${job.preStart}
            end script
          '' else ""}
          
          ${if job.script != "" && job.exec != "" then
              abort "Job ${job.name} has both a `script' and `exec' attribute."
            else if job.script != "" then
              ''
                script
                  ${job.script}
                end script
              ''
            else if job.exec != "" then
              ''
                exec ${job.exec}
              ''
            else
              # Simulate jobs without a main process (which Upstart 0.3
              # doesn't support) using a semi-infinite sleep.
              ''
                exec sleep 1e100
              ''
          }

          ${if job.respawn && !job.task then "respawn" else ""}

          ${if job.postStop != "" then ''
            stop script
              ${job.postStop}
            end script
          '' else ""}

          ${job.extraConfig}
        '';

    in
      pkgs.runCommand ("upstart-" + job.name)
        { inherit (job) buildHook; inherit jobText; }
        ''
          eval "$buildHook"
          ensureDir $out/etc/event.d
          echo "$jobText" > $out/etc/event.d/${job.name}
        '';

        
  jobs =
    [ upstart ] # for the built-in logd job
    ++ map (job: job.upstartPkg) (attrValues config.jobs);

    
  # Create an etc/event.d directory containing symlinks to the
  # specified list of Upstart job files.
  jobsDir = pkgs.runCommand "upstart-jobs" {inherit jobs;}
    ''
      ensureDir $out/etc/event.d
      for i in $jobs; do
        if ln -s $i . ; then
          if test -d $i; then
            ln -s $i/etc/event.d/* $out/etc/event.d/
          fi
        else
          echo Duplicate entry: $i;
        fi;
      done
    ''; # */


  jobOptions = {

    name = mkOption {
      # !!! The type should ensure that this could be a filename.
      type = types.string;
      example = "sshd";
      description = ''
        Name of the Upstart job.
      '';
    };

    buildHook = mkOption {
      type = types.string;
      default = "true";
      description = ''
        Command run while building the Upstart job.  Can be used
        to perform simple regression tests (e.g., the Apache
        Upstart job uses it to check the syntax of the generated
        <filename>httpd.conf</filename>.
      '';
    };

    description = mkOption {
      type = types.string;
      default = "(no description given)";
      description = ''
        A short description of this job.
      '';
    };

    startOn = mkOption {
      # !!! Re-enable this once we're on Upstart >= 0.6.
      #type = types.string; 
      default = "";
      description = ''
        The Upstart event that triggers this job to be started.
        If empty, the job will not start automatically.
      '';
    };

    stopOn = mkOption {
      type = types.string;
      default = "shutdown";
      description = ''
        The Upstart event that triggers this job to be stopped.
      '';
    };

    preStart = mkOption {
      type = types.string;
      default = "";
      description = ''
        Shell commands executed before the job is started
        (i.e. before the job's main process is started).
      '';
    };

    postStop = mkOption {
      type = types.string;
      default = "";
      description = ''
        Shell commands executed after the job has stopped
        (i.e. after the job's main process has terminated).
      '';
    };

    exec = mkOption {
      type = types.string;
      default = "";
      description = ''
        Command to start the job's main process.  If empty, the
        job has no main process, but can still have pre/post-start
        and pre/post-stop scripts, and is considered "running"
        until it is stopped.
      '';
    };

    script = mkOption {
      type = types.string;
      default = "";
      description = ''
        Shell commands executed as the job's main process.  Can be
        specified instead of the <varname>exec</varname> attribute.
      '';
    };

    respawn = mkOption {
      type = types.bool;
      default = true;
      description = ''
        Whether to restart the job automatically if its process
        ends unexpectedly.
      '';
    };

    task = mkOption {
      type = types.bool;
      default = false;
      description = ''
        Whether this job is a task rather than a service.  Tasks
        are executed only once, while services are restarted when
        they exit.
      '';
    };

    environment = mkOption {
      type = types.attrs;
      default = {};
      example = { PATH = "/foo/bar/bin"; LANG = "nl_NL.UTF-8"; };
      description = ''
        Environment variables passed to the job's processes.
      '';
    };

    extraConfig = mkOption {
      type = types.string;
      default = "";
      example = "limit nofile 4096 4096";
      description = ''
        Additional Upstart stanzas not otherwise supported.
      '';
    };

  };

  
  upstartJob = {name, config, ...}: {
    options = {
      upstartPkg = mkOption {
        default = makeJob config;
        type = types.uniq types.package;
        description = ''
          Upstart package which contains upstart events inside
          <filename>/etc/event.d/</filename>.  The default value is
          generated from other options.
        '';
      };
    };

    config = {
      # The default name is the name extracted from the attribute path.
      name = mkDefaultValue (
        replaceChars ["<" ">" "*"] ["_" "_" "_name_"] name
      );
    };
  };

in
  
{

  ###### interface
  
  options = {

    jobs = mkOption {
      default = {};
      description = ''
        This option defines the system jobs started and managed by the
        Upstart daemon.
      '';
      type = types.attrsOf types.optionSet;
      options = [ jobOptions upstartJob ];
    };
  
    tests.upstartJobs = mkOption {
      internal = true;
      default = {};
      description = ''
        Make it easier to build individual Upstart jobs. (e.g.,
        <command>nix-build /etc/nixos/nixos -A
        tests.upstartJobs.xserver</command>).
      '';
    };
    
  };


  ###### implementation
  
  config = {

    system.build.upstart = upstart;

    environment.etc =
      [ { # The Upstart events defined above.
          source = "${jobsDir}/etc/event.d";
          target = "event.d";
        }
      ];

    # !!! fix this
    tests.upstartJobs = { recurseForDerivations = true; } //
      builtins.listToAttrs (map (job: {
        name = removePrefix "upstart-" job.name;
        value = job;
      }) jobs);
  
  };

}